I say ROTJ marked the point George Lucas decided that Obi-Wan’s robes from the first movie were the official Jedi vestments, as evidenced by Anakin’s ghost, which retroactively creates a problem.
Kinda, but also Anakin is from Tatooine. But mainly, it works for the audience best. If they had seen Anakin in some sleek Luke’s black uniform or samurai like wardrobe, he wouldn’t have looked nearly as friendly and kind than old Ben’s familiar comfortable robes.
Yeah, that’s the reasoning I go with also. Anakin assumed a form he knew his son would find comforting and familiar.